It is common practice for traction vehicles, such as transit vehicles, locomotives, and off-highway trucks, to be powered by either direct current (DC) or alternating current (AC) electric motors. The power developed by such motors may be as high as 2600 horsepower (HP) per motor. Such high HP motors require commensurate high, controlled electric power. For example, a nominal power requirement may be 750 volts at 1000 amperes during a propulsion mode of operation. During electrical braking of the traction vehicle, the motors may be operated as generators and produce even higher voltage and current. In the case of an AC electric motor operating as a generator, the peak voltage may routinely exceed 1000 volts.
The power control systems for such electric traction motors typically utilize power semiconductor devices, such as gate turn-off thyristors, for controlling power flow to and from the motors. Due to the magnitude of power being controlled, there is a significant amount of heat that must be dissipated by the semiconductor devices. Such heat is generally handled by mounting the semiconductors on relatively large heat sinks. The heat sinks are massive metal conductors having good thermal characteristics and sufficient surface area to dissipate sufficient heat to maintain the semiconductors within their operating temperature limits.
It is desirable, for safety reasons, that such large heat sinks be maintained at electrical ground potential. Conversely, the semiconductors are connected to high potentials. Accordingly, some form of electrical insulation must be provided between the semiconductors and the heat sink. This insulation is conventionally sheet material having good thermal characteristics, since it is positioned between the semiconductors and the heat sink, and one such material is available under the trade name of Chotherm. Such material is believed to be boron nitride material with a silicon binder. The semiconductors are clamped onto the heat sink with the insulation material clamped therebetween. One problem with this arrangement is that any path along the surface of the insulation sheet between the semiconductors and the heat sink must be established to have a length to prevent voltage breakdown due to creepage, where creepage is defined as the conduction of electricity across the surface of an insulator or dielectric. The creep distance, i.e., the shortest distance across the insulator surface between two conductors of different potential, is established empirically and, in an exemplary system, may require as much as two inches to effectively isolate conductors having a difference in potential of 1000 volts. This creep distance requirement has necessitated that enclosures for high power semiconductors be large and cumbersome and utilize large areas of expensive sheet insulation.
Prior art methods of mounting of semiconductors within enclosures has further exacerbated the size requirements for such enclosures. In particular, it is common practice to mount the semiconductors by individually clamping the semiconductors to the heat sink. The semiconductors of this power requirement are preferably packaged in "press packs" in which the cathode and anode terminals are large, flat surfaces on opposite ends of the semiconductors. Since the clamps are necessarily coupled to the heat sink, any calculation of creep distance includes spacing of the clamps from the semiconductors. Individual clamps are especially required where adjacent semiconductors may be clamped at different pressures. Thus, it is also desirable to provide a method and apparatus for mounting semiconductors which reduces the number of clamps while allowing different mounting pressures to be applied by a single clamp.